Transcript Document

Magnum Bulletin ISO-GEN1-11A
HELICAL PILE/ANCHOR CASE HISTORIES
AND ASSOCIATED LESSONS LEARNED
Presented at DFI Helical Foundations Seminar, Las Vegas, Nevada, January 2010
Howard A. Perko, Ph.D., PE
Director of Engineering for Magnum Geo-Solutions, LLC
Chairman, DFI Helical Foundations and Tie-Backs Committee
Author, “Helical Piles: A Practical Guide to Design & Installation”
Copyright © 2010 Howard Perko
All Rights Reserved
Abstract
With the variety of subsurface profiles, diverse geologic conditions, and a myriad
of structures to be supported, the engineer and contractor need as many
resources in their tool belt as possible. Helical piles and helical anchors represent
one of those tools which, when used correctly, perform effectively. Like all piles
and ground reinforcing elements, helical piles and helical anchors used incorrectly
can have less than favorable results. Several case histories with less than
favorable results are presented so the audience can learn the correct application
of helical piles and helical anchors.
Among the case histories are an underpinning project that resulted in a temporary
moratorium on helical piles in New York City, a tie-back wall at a stadium, a sea
wall with poor load test results, excessive settlement of grouted helical piles, and a
residential underpinning project where a worker was unfortunately killed. Among
the lessons learned are improper lateral bracing, torque miscalibration, and design
changes during construction.
Copyright © 2010 Howard Perko
All Rights Reserved
Disclaimer
The case histories include helical piles/anchors manufactured by different companies
collected over 18 years of experience by Howard Perko, Ph.D., P.E. The presentation is not
meant to criticize any particular installer or manufacturer’s systems and is certainly not a
criticism of helical piles/anchors in general. Any interpretation of this sort is purely
accidental. Rather, Dr. Perko has dedicated much of his life to understanding these devices,
is a proponent of them, and believes the best way to promote the industry is to teach about
potential pitfalls in design and application through a discussion of case histories. Some of the
case histories are ongoing cases wherein Dr. Perko has been retained as an expert
consultant and the name and location of the projects has been withheld.
Copyright © 2010 Howard Perko
All Rights Reserved
Outline
I.
Buckled Underpinning in New York
(a case for lateral bracing)
II.
Stadium Soil Nail Wall Failure
(a review of common anchor design mistakes)
III.
Out-of-Spec Sea Wall Anchors
(the importance of torque calibration)
IV.
Down-Drag of Grouted Helical Piles
(failure to account for consolidation)
V.
Collapse During Foundation Repair
(death as a result of undermining)
VI.
Settlement of Apartment Buildings
(designing from flawed load tests and ignoring theory)
VII.
Scaffold Collapse on Helical Piles
(a failure to provide lateral stability)
Copyright © 2010 Howard Perko
All Rights Reserved
Buckled
Underpinning
in New York
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Helical piles were installed to reduce settlement during conventional
concrete underpinning
Footing rotated, piles buckled, and building settled severely
Photo shows supplemental raker bracing system installed with deadmen
piles after settlement occurred
Printed by permission
of NYC DOB
Copyright © 2010 Howard Perko
All Rights Reserved
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Photo shows buckled pile and rotated bracket system
Helical piles were slender shaft, possibly with reinforcing “T-Pipe” sleeves
All piles were installed along the outside of the building
This and a similar failure were catalysts to a nearly 2 yr moratorium on
helical piles in NYC
Printed by permission
of NYC DOB
Copyright © 2010 Howard Perko
All Rights Reserved
Potential Cause of Failure
 Failed to follow an existing code provision to
brace the tops of piles
Per IBC2006: 1807.2.4 Pile or Pier Stability (Similar
Section in NYC BC)
◆ All piles shall be braced for lateral stability
◆ Three or more piles per cap is considered
braced
◆ Piles staggered under a wall are considered
braced
◆ Otherwise provide engineered means of lateral
bracing
Copyright © 2010 Howard Perko
All Rights Reserved

One solution is to
stagger the piles on
both sides of the
foundation wall.
Copyright © 2010 Howard Perko
All Rights Reserved
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Another solution is to brace the wall laterally by doweling into the floor
slab as in this photo.
Square plates along the base of the wall are concrete anchors extending
into the building slab.
All piles were designed to withstand buckling while excavation was open.
Copyright © 2010 Howard Perko
All Rights Reserved

Another solution is to install tie-back anchors to brace the
foundation at the top of the piles.
Copyright © 2010 Howard Perko
All Rights Reserved
From Perko (2009)
Helical Piles, Wiley, NY
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On smaller structures, bracing can be achieved internally.
Copyright © 2010 Howard Perko
All Rights Reserved
Stadium Soil Nail
Wall Failure
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3 rows of helical soil nails
Approximately 15-foot cut between
bleachers
Reportedly a progressive failure
Copyright © 2010 Howard Perko
All Rights Reserved
Potential Causes of Failure
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Soil nails lacked continuous bonding
Soil nail spacing too far
Soil nail length too short
Bearing capacity failure
Insufficient lapping of reinforcing steel
Poor drainage
Copyright © 2010 Howard Perko
All Rights Reserved
Typical soil
“anchor” wall
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Purpose is to hold back facing
Rigid facing that spans between
anchors
Post-tension anchors
Helical bearing plates located
far outside failure plane
Anchors at angle from
horizontal
Foundation resists downward
component of anchor force
Large anchor spacing
Higher capacity, longer anchors
Drainage behind wall
From Perko (2009)
Helical Piles, Wiley, NY
Copyright © 2010 Howard Perko
All Rights Reserved
Typical soil
“nail” wall
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Purpose is to build a reinforced
soil block
Minimal spacing that simply
resists raveling
Generally not post-tensioned
Helical bearing plates located
within failure plane
Anchors often horizontal
Minimal foundation
Small anchor spacing
Low capacity, short anchors
Drainage behind wall
From Perko (2009)
Helical Piles, Wiley, NY
Copyright © 2010 Howard Perko
All Rights Reserved
Out-of-Spec Sea
Wall Anchors
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1 row of approximately 35’ long helical tie-backs
Continuous torque readings during installation
Approximately 15-foot cut to removed failed timber crib wall
All 18 helical anchors failed proof test
Supplemental anchors based on same design methods, installed by
same crew using different torque motor passed proof tests
Copyright © 2010 Howard Perko
All Rights Reserved
Potential Cause of Failure
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Flawed torque motor
calibration
Copyright © 2010 Howard Perko
All Rights Reserved
Down-Drag of
Grouted Helical
Piles
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DESCRIPTION
DEPTH (FT)
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Grouted pull-down piles
Houses settled several
inches
Approximately 7-feet of
site grading fill
Piles bottomed in soft
clay, grouted full depth
GRAPHIC LOG
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0.5 6" TOPSOIL
SITE GRADING FILL
7.0
10
CLAY
20
BOTTOM OF BORING
30
Copyright © 2010 Howard Perko
All Rights Reserved
Potential Cause of Failure
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Down-drag of site
grading fill on grouted
pile
From Perko (2009)
Helical Piles, Wiley, NY
Copyright © 2010 Howard Perko
All Rights Reserved
Collapse During
Foundation Repair
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An addition was being constructed on the back of this home
The existing back wall experienced settlement
A foundation repair contractor was called out to install several piles along
the back wall
Copyright © 2010 Howard Perko
All Rights Reserved
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During installation of piers, the wall suddenly collapsed, tragically killing
one of the workers.
Possible causes of accident may include undermining of existing footing,
failure to account for eccentricity exerted on foundation by underpinning
system, and lack of lateral bracing of foundation wall
Copyright © 2010 Howard Perko
All Rights Reserved
Settlement of
Apartment Buildings
Approximately 140 helical piles installed to support several 5-story
apartment buildings
 Design load of helical piles= 45 to 65 tons
 Immediately after construction, several of the buildings exhibited
settlement on the order of 4”
 Three of the buildings had to be demolished and re-built. The others were
repaired by underpinning.
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Copyright © 2010 Howard Perko
All Rights Reserved
Potential Causes of
Failure
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Non-conforming helix
invalidates capacity to torque
readings
Insufficient bearing area; relied
on torque alone and did not
check theoretical bearing
capacity
Over-reliance on load tests
Copyright © 2010 Howard Perko
All Rights Reserved

Flawed load test results, actual measured deflection
impossible because it is less than calculated elastic
shortening of the helical pile shaft
Copyright © 2010 Howard Perko
All Rights Reserved
Scaffold Collapse on
Helical Piles
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Approximately 4-story tall,
temporary, stand-alone scaffold
towers were erected to construct an
elevated concrete walkway
Each scaffold tower was supported
on 8 helical piles
During concrete placement, scaffold
gave-way killing one worker and
injuring 18 others
Example Frame Scaffolding (Note:
This is not the actual scaffold that
collapsed. Due to confidentiality,
actual scaffold cannot be shown.)
Copyright © 2010 Howard Perko
All Rights Reserved
Potential Causes of
Failure
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Structural issues associated
with scaffold design (lack of
lateral-torsional bracing)
Foundation and structure relied
upon each other for stability;
together they were unstable
No lateral capacity specification
for the piles
Square-shaft helical piles had
negligible lateral capacity
Copyright © 2010 Howard Perko
All Rights Reserved
Thank You